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61.
62.
Ohne Zusammenfassung 相似文献
63.
Profiles of the K line of Caii are computed for a two component solar chromosphere, chosen to simulate with a simple geometry the chromospheric supergranular network. Each component rises above the BCA photosphere, the boundary component representing the bright network with a sharp temperature rise and the cell component representing the darker region with an extended temperature minimum. Theoretical intensity profiles of the Can K core, calculated as weighted averages over the projected areas of the components, are produced for = 0.6 and 0.3. The line source function and the optical depth are obtained from a self-consistent treatment of the steady state and radiative transfer equations, with complete redistribution assumed for scattering in the line. The atomic model consists of two bound levels and a continuum. It is found that a 4600 K minimum can lead to the successful theoretical prediction of the observed limb darkening and 4300 K radiation temperature of the K1 feature only when very large values of turbulent velocity are assumed to exist in the cell region.Publications of the Goethe Link Observatory, Indiana University, No. 95. 相似文献
64.
David W. Kelley Stefanie A. Brachfeld Edward A. Nater Herbert E. Wright Jr. 《Journal of Paleolimnology》2006,35(1):193-206
Sediments from Lake Pepin on the Mississippi River, southeastern Minnesota, are used as provenance tracers to assess variations
in hydrology and sediment-transport during the middle Holocene. Three rivers contribute sediment to Lake Pepin, and each catchment
is characterized by a distinctly different geologic terrain. The geochemical fingerprint for each drainage basin was determined
from the elemental composition of heavy minerals in the silt-sized fraction of modern sediment samples. Down-core elemental
abundances were compared with these fingerprints by use of a chemical-mass-balance model that apportions sediment to the source
areas. We observed a decreased contribution from the Minnesota River during the interval ~6700–5500 14C yr BP, which we attribute to decreased discharge of the Minnesota River, likely controlled by a combination of precipitation,
snow melt, and groundwater input to the river. This hydrologic condition coincides with the mid-Holocene prairie period recorded
by fossil pollen data. The occurrence of this feature in a proxy record for hydrologic variations supports the hypothesis
that the mid-Holocene prairie period reflects drier conditions than before or after in midwestern North America. 相似文献
65.
Lorenzo Iorio Herbert I. M. Lichtenegger Matteo Luca Ruggiero Christian Corda 《Astrophysics and Space Science》2011,331(2):351-395
Recent years have seen increasing efforts to directly measure some aspects of the general relativistic gravitomagnetic interaction
in several astronomical scenarios in the solar system. After briefly overviewing the concept of gravitomagnetism from a theoretical
point of view, we review the performed or proposed attempts to detect the Lense-Thirring effect affecting the orbital motions
of natural and artificial bodies in the gravitational fields of the Sun, Earth, Mars and Jupiter. In particular, we will focus
on the evaluation of the impact of several sources of systematic uncertainties of dynamical origin to realistically elucidate
the present and future perspectives in directly measuring such an elusive relativistic effect. 相似文献
66.
Wolfram Neutsch Herbert Kalisch Horst Fichtner S. Ranga Sreenivasan Maurice Shevalier 《Astrophysics and Space Science》2003,288(3):391-408
The study of the expansion of the solar wind out of a system of coronal holes is continued. To this end, we consider the numerical
integration of partial differential equations for problems with icosahedral symmetry, in general. First, employing Weyl theory,
orbifold coordinates are introduced. Second, the icosahedral coordinates are discussed in detail. Third, following an analysis
of the properties of these coordinates and the derivation of a few expressions useful for grid construction, various alternatives
for the distribution of lattice points required for numerical integration are considered. A comparison of these numerical
grids motivates the choice of a specific grid optimized for the numerical integration carried out in the accompanying paper
by Kalish et al.(2002).
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
67.
68.
The unexpectedly large scale height of Io's ionosphere (Kliore, A., et al., 1975, Icarus24, 407–410) together with the relatively large molecular weight of the likely principal constituent, SO2 (Pearl, J., et al., 1979, Nature280, 755–758), suggest a high ionospheric temperature. Electrical induction in Io's ionosphere due to the corotating plasma bound to the Jovian magnetosphere is one possible source for attainment of such high temperatures. Accordingly, unipolar induction models were constructed to calculate ionospheric joule heating numerically. Heating rates produced by highly simplified models lie in the range 10?9 to 10?8 W/m3. These heating rates are lower than those determined from uv photodissociative heating models (Kumar, S., 1980, Geophys. Res. Lett.7, 9–12) at low levels in the ionosphere but are comparable in the upper ionosphere. The low electrical heating rate throughout most of the ionosphere is due to the power limitation imposed by the Alfvén wings which complete the electrical circuit (Neubauer, F.M., 1980, J. Geophys. Res.85, 1171–1178). Contrary to the pre-Voyager calculations of Cloutier, P. A., et al. (1978, Astrophys. Space Sci.55, 93–112), our numerical results show that the J × B force density due to unipolar induction currents in the ionosphere is much less than the gravitational force density when the combined mass of the neutral species is included. The binding and coupling of the ionosphere is principally due to the relatively dense (possibly localized) neutral SO2 atmosphere. In regions where the ions and neutrals are collisionally coupled the ionosphere will not be stripped off by the J × B forces. However at a level above that (to which the ions move by diffusion only) the charged species would be removed. Thus there appears to be no need to postulate the existence of an intrinsic Ionian magnetic field as suggested by Kivelson, M. G., et al. (79, Science 205, 491–493) and Southwood, S. J., et al. (1980, J. Geophys. Res., in press) in order to retain the observed ionosphere. 相似文献
69.
70.
A densely populated soft-sediment community was experimentally analysed for interspecific competition for space in laboratory experiments. No interspecific competitive interactions leading to a decrease in survivorship could be documented over a five-week period. The four most abundant species all coexisted in laboratory microcosms. The two tube-buiding species, the tanaid Leptochelia dubia and the polychaete Rhynchospio arenincola, could not exclude the mobile bivalve Transennella tantilla or the burrowing amphipod Paraphoxus spinosus. None of the species increases its emigration frequency when other species are present. Paraphoxus is found to be a browsing predator on Phynchospio but inflicts no detectable mortality. The coexistence of these species is permitted by virtue of the fact that none of the species can alter the soft-sediment habitat sufficiently to make the habitat unsuitable for the other species. 相似文献